This application claims the priority of German Application Nos. 198 23 395.7, filed May 26, 1998 and 198 34 128.8, filed Jul. 29, 1998, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a service brake for a vehicle having two brake circuits which are acted upon by pressurized fluid via a vacuum brake booster and a dual brake master cylinder.
Service brakes of this type have been used for some time in virtually all passenger cars and smaller utility vehicles. In this case, the vacuum brake boosters are designed such that, in the event of a failure of the vacuum, a direct coupling is possible between a piston connected with a brake pedal and the piston of the dual brake master cylinder. As a result of this coupling, the vehicle can still be braked in this emergency situation.
A problem in such emergency situations is that only limited brake pedal forces can be applied by a person operating the brake pedal. This is particularly true if, for reasons of comfort, the pedal travels were dimensioned to be short in the case of a power-assisted braking operation. For this reason, in emergency situations, only vehicles up to approximately 6 tons total weight can be braked safely. For heavy vehicles, such an emergency braking function of the service brake is no longer sufficient.
It is an object of the present invention to further develop a service brake for a vehicle of the above-mentioned type such that in the event of a failure of the vacuum brake booster, a reliable braking of the vehicle is possible even in the case of heavier vehicles. In particular, braking is to be permitted which meets the minimum legal requirements concerning the deceleration of the vehicle during a braking operation.
In the case of a service brake for a vehicle of the above-mentioned type, this object is achieved in that at least one triggerable, switchable device is provided for changing the hydraulic pressure intensification by which, in the event of a failure of the vacuum brake booster, the pressure of the fluid is increased in each of the two brake circuits.
By means of the triggerably switchable device for changing the hydraulic pressure intensification, which increases the pressure of the fluid in the event of a failure of the vacuum brake booster in both brake circuits, an increase of the pressure existing in both brake circuits is permitted in an emergency situation in a particularly simple manner.
As far as the construction of the device for changing the hydraulic pressure intensification is concerned, purely in principle, the most varied embodiments are described here. In an advantageous embodiment, the at least one device for changing the hydraulic pressure intensification is in each case a step piston cylinder unit which is connected behind the dual circuit brake booster in each of the two brake circuits. This embodiment has the particularly important advantage wherein a retrofitting of existing service brakes is possible without changing the brake booster and/or the dual brake master cylinder.
In this case, it is preferably provided that each step piston cylinder unit has a step piston which is stepped on the input side.
The switching-over of the step piston cylinder units advantageously takes place by an electrically triggerable valve.
The valve is preferably a 3/2-way valve by which, in its one switching position, an input volume of the step piston cylinder unit can be connected with an input-side ring volume. In its other switching position, an output volume can be connected with the input-side ring volume of the step piston cylinder unit. Thus, by a switch-over of the ring volume between the input-side input volume and the larger output-side output volume, an increase of the hydraulic pressure intensification can be achieved in a particularly simple manner.
In the case of another very advantageous embodiment, the at least one device for changing the hydraulic pressure intensification is a step piston cylinder unit which is connected in front of the dual brake master cylinder and which can be switched over by way of an electrically triggerable valve. In this embodiment, only one step piston cylinder unit is still required, whereby the technical expenditures and costs can be reduced.
A particularly advantageous embodiment has the step piston cylinder unit part of the dual brake master cylinder. This permits a further minimizing of the required number of components of the service brake of the vehicle.
The relationship of the fluidically active surfaces of the step piston cylinder unit can be constructed such that the effective surface of the non-stepped portion of the step piston cylinder unit is approximately as large as the effective surface of the dual brake master cylinder. These surfaces are larger than the surface of the stepped portion of the step piston cylinder unit so that a hydraulic pressure intensification becomes operative.
In another advantageous embodiment, the effective surface of the dual brake master cylinder is approximately as large as the effective surface of the stepped portion of the step piston cylinder unit. Both effective surfaces are smaller than the fluidically effective surface of the non-stepped portion of the step piston cylinder unit. In this embodiment, only one bore is required for manufacturing the dual brake master cylinder and the stepped portion of the step piston cylinder unit which is part of the dual brake master cylinder. This considerably reduces the manufacturing expenditures and manufacturing costs.
The valve is preferably a 3/2-way valve by which, in one switching condition, an output volume can be connected with an output-side ring volume of the step piston cylinder unit and by which, in its other switching position, the output-side ring volume can be connected with an input volume. In a simple manner, this again permits increasing the hydraulic pressure intensification.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.